This invention relates to PEM/SPE fuel cells and more particularly to a method of conditioning such fuel cells for inactivity (e.g. storage) at subfreezing conditions.
Fuel cells have been proposed as a power source for many applications. So-called PEM (proton exchange membrane) fuel cells [a.k.a. SPE (solid polymer electrolyte) fuel cells] potentially have high energy and low weight, and accordingly are desirable for mobile applications (e.g. electric vehicles). PEM/SPE fuel cells include a xe2x80x9cmembrane electrode assemblyxe2x80x9d (a.k.a. MEA) comprising a thin proton-transmissive, solid-polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements that serve as current collectors for the anode and cathode and contain channels/grooves therein forming a so-called xe2x80x9cflow fieldxe2x80x9d on the faces thereof for distributing the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode.
PEM/SPE fuel cells are typically H2xe2x80x94O2 fuel cells wherein hydrogen is the anode reactant (i.e. fuel), and oxygen is the cathode reactant (i.e. oxidant). The oxygen can be either in a pure form or diluted with nitrogen (e.g. air), and the hydrogen can either be in a pure form or derived from the reformation of methanol, gasoline or the like. The solid polymer membranes are typically made from ion exchange resins such as perfluoronated sulfonic acid. One such resin is NAFION(trademark) sold by the DuPont Company. Such membranes are well known in the art and are described in U.S. Pat. No. 5,272,017 and 3,134,697 as well as in the Journal of Power sources, Vol. 29, (1990), pages 367-387, inter alia. The anode and cathode typically comprise finely divided catalytic particles either alone or supported on the internal and external surfaces of carbon particles and have proton conductive resin intermingled therewith.
Commercially available solid polymer membranes all require some degree of humidification to be effective. Hence a humidifier is typically provided somewhere in the in the fuel cell system to supply moisture to the cells. Moreover, the current-producing fuel cell reaction (i.e. H2+O2xe2x86x92H2O) forms water in situ within the cell during normal operation thereof. If allowed to freeze, the water in the cells forms ice which (1) can plug the flow channels and prevent any reactant gas from passing therethrough, (2) can damage the polymer membrane, and (3) can exert deleterious pressures within the cell(s) resulting from the expansion of the water during freezing. As long as the fuel cell is operating (i.e. producing current) or is otherwise heated, ice formation is not a problem. However during shut down, storage, or other inactivity of the fuel cell under freezing conditions damaging ice can form.
The present invention overcomes the problem associated with ice formation in inactive/unheated fuel cells that are subjected to freezing conditions.
The present invention contemplates a method of conditioning a PEM/SPE fuel preparatory to its being rendered inactive at subfreezing conditions. More specifically, the present invention contemplates substantially dehydrating the fuel cell before it can freeze by evacuating the flow field(s) of the fuel cell(s) with a vacuum that is sufficient to evaporate and remove enough water from the fuel cell(s) as to prevent damage thereto due to freezing. Preferably, evacuation of the fuel cell occurs when the cell stack has a temperature of at least about 20xc2x0 C. In this regard, the water is more easily evaporated with a lesser vacuum from a warm fuel cell than from a cooler one. Most preferably, the fuel cell stack is normally operated at an elevated temperature (e.g. about 80xc2x0 C.), and is evacuated to remove the water shortly after it is shut down and still warm (i.e. at least about 50xc2x0 C.) from its operation.